A diffusion bonding technology for bonding steel members has been known (see, e.g. Japanese Patent Publication No. 62-045020).
In Japanese Patent Publication No. 62-045020, steel members are bonded with an insert material sheet by heating the entire joint to a temperature of about 1200° C. or higher, and the insert material sheet is prepared by mixing a carbon powder with a binder containing an acrylic resin and a solvent and by forming the mixture into a sheet shape. When the carbon is diffused into a connection portion, the melting point of the steel material to be bonded is lowered. Therefore, the steel members can be bonded at a relatively low temperature.
The technology of Japanese Patent Publication No. 62-045020 is a brazing technology, whereby it takes about 5 hours to bond the steel members. In the brazing technology, a brazing material is sandwiched between the steel members, and the resultant is introduced into a brazing furnace, heated from an atmospheric temperature to a temperature of 1200° C. or higher, and maintained at a temperature of 1200° C. or higher. The acrylic resin is heated and evaporated in the heating process, while the carbon powder remains in the product.
During the process of heating from the atmospheric temperature (25° C.) to 1200° C., a part of the carbon powder is diffused into the steel material to be bonded while the acrylic resin is evaporated.
Therefore, when the temperatures of the steel members reach 1200° C. or higher, the amount of the carbon powder is often insufficient for the bonding. When the amount of the carbon powder is insufficient, the diffusion reaction proceeds insufficiently, and the steel members are not bonded sufficiently in part.
The supply amount of the insert material may be increased to compensate for the shortage of the carbon powder. However, this increase is costly and may result in excess of carbon. The excess of carbon leads to a defect such as embrittlement of the connection portion.
Thus, there has been a demand for developing a bonding technology capable of preventing carbon from being diffused during a heating process.